EC inspection is commonly used in NDT/NDI applications to detect flaws in surfaces of manufactured components fabricated out of conductive materials, such as steel bars, tubes and pipes. EC is often used to inspect components for automotive, aeronautic and energy industries. Over the years, EC sensors have been designed with different configurations and patterns to suit for different applications.
Various EC systems have heretofore been provided for the detection of cracks and or other flaws in a part under test. In general, such systems include field producing means such as a coil connected to an AC source to generate EC's in a part and a sensing means to sense the field produced by the EC's. The sensing means may be a separate coil, a Hall probe, or any other field responsive device, or the coil of the field-producing means may also be used to sense the EC-induced field, by measuring the effective impedance thereof.
In such prior systems, difficulties are encountered due to the changes in conductivity and permeability of the part under test and also due to the variations in spacing between the test coil or probe and the surface of the part, and variation in surface conditions. It has been possible to reduce the effect of variations in spacing by certain arrangements such as by the use of impedance networks and by adjustment of operating frequency. Such arrangements, however, have not overcome the sensitivity to conductivity and permeability changes. To reduce the effect of conductivity and permeability changes, differentially connected coils have been used. However, such arrangements have been insensitive to defects common to the differentially connected coils.
Background art has evolved over the years with the general object of overcoming the disadvantages described above for EC testing systems and providing systems which are very sensitive to defects while being insensitive to variations in other physical characteristics of a part under test and variations in the physical relation of a test probe to the part. U.S. Pat. No. 3,495,166 is incorporated by reference as the example for background art described below.
In accordance with an important feature of the background art, an EC system is provided which includes field-sensing means for sensing fields produced by EC's in two regions having substantially the same spatial relation to a surface of the part and having a substantial angle therebetween with detector means being provided for detecting differences between the fields produced in the two regions. It should be noted that the sensing regions of the field-sensing means are orthogonal to the emitted magnetic field regions of field-producing means. Accordingly, in the absence of a defect that will disrupt the direction of the EC flow imparted by the field-producing means, the magnetic field resulting from the EC flow will also be orthogonal to the field-sensing means and will consequently not be sensed. With this arrangement, a high degree of sensitivity is obtained with respect to flaws having different orientations with respect to the sensing regions, while being insensitive to changes in a) conductivity, b) permeability, c) irregular surface finishes and d) to changes in the spacing of the part. This insensitivity stems from the fact that properties a, b, c and d affect predominantly the magnitude of the EC flow and resulting magnetic field, but not the direction.
It is found that almost all defects which are of interest in the testing of a part have a dimension which is greater in one direction than in another and with a substantial angle being provided between the sensing regions, a high degree of sensitivity to significant types of defects is obtained. At the same time, the sensing regions can be quite close together so as to obtain extremely low sensitivity to variations in spacing or surface conditions, while also obtaining very low sensitivity to changes in conductivity and permeability.
According to another important feature of the background art, the sensing regions are crossed to intersect at mid-points thereof so that the area of the part which is inspected is minimized and so that the sensing regions always have the same physical relationship to the part being inspected.
According to a specific feature of the background art, the angle between the sensing regions is approximately 90 degrees, to obtain maximum sensitivity to defects.
According to another specific feature of the background art, the sensing regions are relatively long and narrow with transverse dimensions equal to a small fraction of the long dimension thereof, to obtain high resolution and to facilitate detection and location of narrow cracks within a part.
In accordance with a further feature of the background art, a pair of coils are used which are located in planes generally transverse to the surface of the part.
In certain of the arrangements according to the background art, the pair of coils are used as part of the field-producing means by connection thereof to an AC source. The same coils may be used as part of the sensing means, or may be used only in the sensing means with another coil or coils being used in the field-producing means. In one arrangement, the field-producing means comprise a coil having an axis generally parallel to a line at the intersection of the planes of a pair of coils used in the sensing means.
In accordance with an important feature of the background art, the coils have matched inductances and resistances, to obtain an accurate balance and to minimize sensitivity to conductivity and permeability variations and sensitivity to changes in the spacing between coils and the test part.
In one arrangement in which the same pair of coils are both used in field-producing and field-sensing means, a bridge circuit is provided having two branches each having two legs with the two branches being connected to an AC voltage source. The pair of coils forms two legs of the bridge circuit while impedance means form the other two legs of the bridge circuit and detector means are provided connected between the junction of the legs of one of the branches and the junction of the legs of the other of the branches. This arrangement further facilitates the attainment of an accurate balance and minimizes sensitivity to conductivity and permeability changes and changes to spacing.
With only one pair of coils, it is possible to miss defects located exactly along an angle intersecting the angle between the coils. Although this deficiency is not usually serious, it can be obviated by the provision of a second pair of coils in planes generally transverse to each other and at angles to the planes of the first pair of coils.
The 3D orthogonal sensor topology described above provides many benefits; however, a few drawbacks have been known to bring limitations to its usage. One such drawback is that, with coils wound onto a cube or cross-shaped core, the sensor is inevitably bulky, which limits the space it can access during inspections. Another drawback is that the fabrication of this sensor largely depends on having the coils manually wound onto the cubes or the cross-shaped cores. The fabrication is labor intensive and costly.
With the advances of printed circuit board (PCB) technologies over the last decades, it is now possible to manufacture some EC sensors with certain coil configurations on a thin, sometimes flexible, support. Significant benefit with the use of PCB technologies to manufacture EC array probes include reduced manufacturing cost, increased sensor flexibility and increased reproducibility. An example of such a probe is described in U.S. Pat. No. 5,389,876.
A drawback of currently available EC sensors or probes made from printed circuit boards is that they are limited to simply mapping the two-dimensional (2D) shape of the prior art coils that are wound on a plane that is approximately parallel to the inspected surface. This is because the printed circuit board is essentially a 2D structure. However, challenges remain in PCB manufacturing for some coil configurations such as used in the orthogonal sensors with a 3D structure.
The use of solid state magnetic field sensors such as anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR), combined with the printed circuit board technologies, made it possible to obtain probes with EC responses similar to the conventional orthogonal sensor. An example of this is shown in a patent publication US2005-0007108. In this publication, a flat winding coil generates ECs in the component under test while a GMR field sensor array picks up the orthogonal magnetic field generated when a defect disturbs the ECs. While this technology benefits some applications, it is unable to provide a fully flexible probe because the AMR and GMR sensors are discrete components on the PCB. There are also many limitations intrinsic to AMR and GMR sensors such as the risk of saturation and the need for magnetic biasing, both of which presents undesirable concerns in an industrial environment.
Accordingly, it is desirable to provide a method for emulating the EC effect of a 3D EC sensor structure using a 2D winding configuration, which is suitable to be fabricated using the current printed circuit board technologies.
It would also be desirable to provide a means for building an EC array probe including sensors that behave in the manner described for the 3D orthogonal background art sensors with the printed circuit board technology.